Tunable cavity resonator



Jan. 14, 1958 w'. P. BENNETT TUNABLE CAVITY RESONATOR Filed Sept. 22,1955 TUNABLE CAVITY RESONATOR Wilfred P. Bennett, Lancaster, Pa.,assignor to Radio Corporation of America, a corporation of DelawareApplication September 22, 1955, Serial No. 535,845

AThe terminal fifteen years of the term of the patent to be granted hasbeen disclaimed 8 Claims. (Cl. 315-41) This invention relates to atunable cavity resonator including a gaseous or liquid iluid, and moreparticularly to a tunable input or output cavity resonator circuit for avacuum tube wherein tuning is accomplished by varying the pressuredifference of the Huid on both sides of a tuning element in the cavityresonator.

The amount of power which can be obtained from an ultra-high frequencyamplifier, such as may be employed in a television or radar transmitter,is often limited by arcing between points in the cavity resonatorcircuit across which high potential dilferences exist. It may beimpossible to increase the mechanical clearances enough to avoid arcingat the desired high power output level because the physical dimensionsof a cavity for use at ultrahigh frequencies must be kept small forelectrical reasons. The power handling capacity of a resonant cavity,such as the output cavity resonator circuit of a vacuum tube, can beincreased by pressurizing the cavity. It the cavity is pressurized,conventional constructions for tuning the cavity are not suitable.

A general object of this invention is to provide an improved pressurizedand tunable cavity resonator construction having high power handlingcapacity.

Another object is to provide an improved tunable vacuum tube input oroutput circuit having very high power handling capacity.

A construction illustrative of the invention comprises a coaxial lineresonant cavity having one end sealed by connection to the electrodecontact rings of a vacuum tube, and having the other end sealed by anend wall. The cavity resonator may be coupled to the output electrodesof the vacuum tube for use as the output circuit of the vacuum tube. Anoutput coupling from the cavity resonator is arranged to include apressure seal. Tuning of the cavity resonator is accomplished by meansof an axially movable tuning element in the cavity resonator. Acompressed gas from a source is applied thru conduits to opposite sidesof the tuning element in the cavity resonator. The tuning element ismoved by varying the pressure on the two sides of the tuning element.After the cavity resonator is accurately tuned to the desired frequency,the gas pressures on both sides of the tuning element are equalized andthe resonant cavity is maintained, under gas pressure to increase thepower handling capacity of the cavity. An insulating liquid may beernployed in place of gas.

These and other objects and aspects of the invention will appear fromthe following more detailed description taken in conjunction with theappended drawing, wherein:

Figure l is a representation of a vacuum tube output circuit constructedaccording to the teachings of this invention;

Figure 2 is an enlarged fragmentary view of the system of Figure lillustrating constructional details of the axially movable tuningelement; and

Figure 3 illustrates a source of hydraulic pressures which arentPatented Jan. 14, 1958 may be substituted for the source of compressedgas in the system of Figure l.

The vacuum tube output 'circuit shown in Figures l and 2 of the drawingsincludes a vacuum tube 10 having a cathode contact ring 11 and an anodecontact ring 12. A coaxial line cavity resonator includes an outerconductor 13 connected to the'cathode contact ring l1 thru a pressureseal connection, and a coaxial inner conductor 14 connected to the anodecontact ring l2 thru a flange 15 and a pressure seal. The end of thecavity resonator remote from the vacuum tube l0 is closed by an end wall16. A radio frequency by-passing and direct current isolating capacitor(not shown) should be included at some point in the circuit to prevent adirect current path between the cathode 11 and anode 12. By way ofexample only, the by-pass capacitor may be located between the anode 12and the assembly including conductor 14 and llange 15. Radio frequencyenergy is coupled from the cavity resonator by means of an outputcoaxial line 17 provided with an insulating pressure seal 18.

The cavity resonator is tuned by means of a tuning element 20 which isarranged tor axial movement in the resonant cavity. The tuning element2t) illustrated in the drawing is a generally cup-shaped metallicelement having a lip 21 arranged tor sliding engagement with the tlange15, and having an aperture in the end wall 22 with an edge arranged torsliding engagement with the inner conductor 14 ot the cavity resonator.The constructional details are shown in greater detail in Figure 2 or'tne drawings where it will De seen that sliding electrical contact ismaintained by means ot' metallic spring tingers 23 and 24 and a slidingpressure seal is maintained by means or' nexible plastic u rings 25 andZ6. 'lne tuning element o cooperates with the adjacent portion ot theouter conductor 13 otl the cavity resonator to provide a section ot'relatively low impedance transmission line which is adjust-aule inposition to enect electrical tuning of the cavity resonator. lne cavityresonator may, tor example, have a total electrical lengtn otthree-quarters of a wavelength at the operating trequency. 'the tuningelement 2u may have an axial length on the order ot a quarterwavelengthat the operating trequency.

1t will oe noted trom rigure l ol' the drawings that the space betweenthe outer and inner conductors' i5 and 14 of tne cavity resonator isdivided by tuning element Z0 into two cells designated 3u and 51. lnecell 3i) is connected oy conduits o; and 'o3 to a source 55 ofcompressed gas; and cell 31 is connected by conduits .'56 and al to thesource 3a ot compressed gas. Valves 38 and 39 are provided in theconduits 3.5 and 37, respectively. A common vaive :il is also provided.

Cell au is also connecten oy conduits 32 and 40 to a vacuum pump ai; andcell '51 is also connected by conduits so and Li2 to the vacuum pump 41.A plurality or valves als thru "i6 are provided in tne conduits 40 and42.

ln the operation of the system of Figure 1, coarse tuning ls pertormedoy operating the valves 325, 39, and 43 thru te to eirect movement otthe tuning element 2l) and thereby tune the cavity resonator to thedesired voperating trequency. 1f it is desired to move the tuningelement 2li upwardly, the valves are operated to make the pressure inthe cell 31 greater than the pressure in the cell 3l?. To move thetuning element 20 downwardly, the pressure ditlerential is reversed.

"lne source 35 of compressed gas should preferably provide a gas such assulphur hexatluoride which has a relatively high ionization potential.The vacuum pump 41 is employed to exhaust the air from the cells 30 and31 of the cavity resonator prior to the insertion of gas from the source35. Air is exhausted from the cavity resonator i aecomo by closing vlve"50, and 'by opening valves 43 thru 48. fter the cavityresonator'isexhausted. valves 43 thru 48 are closed. and Valves 38` 39 and 50 areopened to apolv gesunder equal pressure to both Vot` the cells 30 and'31. Then valves 3S and 39 are closed. LFine tuning is then accomplishedby operating ithe'vlves 43'thru 218. For example. if it isdesired'to'move't'h'e tuning element 20 'a 'small amount 'in 'the upwarddirection. the v'alve 43 is opened 'to reduce the pressure-in the 'cell`30'by an amount determined by the volume in conduit'40 betweenlthevalve 'iand 4d. The reduction vin pressure in the cell '30 resultsfrom the "fact that ,the section of conduit between valves 43 'and '44waspreviously evacuated. An additional upward 'movement of 'the 'tuningelement 20 maybe accomplished :by ope'riin'g 'valve 144. Similarly, thetuning'elemeutz may be-moved'downwardly in small amounts "by operatingvalves 218 `and "47 to reduce the 'pressurei'n cell 31. 'fter the tuningoperation vis cornpleted, the valves 3S and 59 are opened to equalizethe pressures in "cells $0 and 31. The -`rriecha'nical frictionbetweenth'e tuningeleme'n't'ZD andthe contacted surfaces `14 and rsmaintains the tuning-dement 20 in its ser position.

Figure 3 illustrates Ya hydraulic device for use in place of lthecompressed gas and vacuum 'devices in the system of Figure '1. Ahydraulic cylinder '53 vand cooperating piston vS4 provide 'hydraulic'pressures to conduits 32 and 36 inthe system off Y'Figure 1. Accordingto this alternate 4form of the invention, the cylinderl S3, the conduits32 and 36, and the cells 30 and 3-1 are filled with 'an 'insulatingliquid such as oil. `lviovem'e'nt ofrtthe'p'iston 54 varies thehydraulicpres'suresin the cells 30 arid 31 to cause an axial movement ofthe tuning 'element zo. 'The insulating liquid in the cavity resonator'permits `operation at much higher powers 'without electrical ar'cingthan can be obtained with air i'n'th'e'c'avity resonator.

What is claimed :is:

1. A tunable circuit comprising a'sealed cavity 'resonator having Walls;a movable `tuningelerrrent in said resonator and having vslidingpressure seals with the walls of said resonator; asource 'of'fluid, twoconduits from said source of -uid to respective opposite sides of saidtuning element in said 'cavity resonator, and means to vary the fluidpressures 'ditferentia'lly in 'said two 'conduits to move said tuningelement,

2. A tunable energy-transferring circuit for a Vacuum tube comprising asealed cavityresonator having Walls coupled to two 'electrodes 'of saidvacuum tube; 'a movable tunin'g element in said resonator and Lhavingsliding pressure seals with the Walls of said resonator, a source ofliuid, two conduits from said Asource of tluid to respective oppositesides of said tuning element in said 'cavity resonator, and means tovary the fluid pressures vdifferentially in said two vconduits to `movesaid tuning element.

3. A vacuum tube circuit 'comprising a vacuum tube, a coaxial linecavity resonator having Yinner and Aouter conductors sealed at one endby a connection to said vacuum tube and sealed at the Aother end by anend Wall; a tuning element positioned between said coaxial conductorsand adapted for axial movement iu said resonator, said tuning elementhaving sliding pressure seals-engaging at least one of said conductors;a source of fluid, two conduits connected from said source of fluid torespective opposite sides of said tuning element` and means to vary thepres- Vsuretiitlerence in said conduits.

4. A vacuum tube circuit comprising a vacuum tube, a coaxial line cavityresonator having inner and outer conductors sealed at one end by aconnection to said vacuum tube and sealed at the other end vby an endwall; a tuning element positioned between said coaxial conductors andadapted for axial vmovement in said resonator, said tuningelementdividing the cavity into two cells and having sliding pressureseals engaging at least vone of saidconductors; a source of uid, twoconduits connected from said source of duid to said -respective cells,and means to vary the pressure difference in said conduits.

5. A lvacuum tube circuit comprising a vaculmtube, a coaxial line cavityresonator having inner and outer conductors sealed at one-end by "aconnection to said vacuum tube and sealed at the other end by an endWall; a tuning element positioned between said coaxial conductors andadapted for axial movement in said resonator, said ituning elementhaving sliding pressure seals engagingat least one 'of 'said conductors;a source of compressed gas, `two conduits connected fromesaid source ofcompressed gas to respective opposite sides of said tuning element, 'avacuum pump, two conduits lconnected from said vacuum pump "torespective opposite'sides of said tuning element, and valves'in saidconduits tocontrol the pressure `diiere'nce on thejtwo'sides of saidtuning element.

6. A tunable renergy-transferring circuit for a vacuum tube comprising asealed Vcavity resonator having walls coupled to two electrodes o'fisaid vacuum tube; a movable tuning element between said Walls andforming two 'cells therein, said-tuni n'g element having slidingpressure seals with the walls of said resonator, Aa source of compressed`gas, a source of vacuum, conduits Iconnecting both said source ofcompressed gas audsaid source'of vacuum to both of'said cells, andvvalves in said conduits to control the pressure Vdifference 'in said`two cells, to thereby move said tuning element.

7. A `tunable energy-transferring circuit for a -vacuum tube comprisinga sealed cavity resonator having walls coupled to two 'electrodes ofsaid vacuum tube; a 'movable tuning velement in said resonator andVhaving sliding pressure seals with the walls-of said resonator; asource of compressed gas, two conduits from said source `of gas torespective 'opposite sides of said tuning element in said resonator, andmeans to vary Athe gas pressures differentially in said twocou'duitstomove said tuning element.

f8. A tunable energy-"transferring circuit for a vacuum tube comprisingza sealed Icavity resonator vhaving walls coupledto vtwo electrodes ofsaid vacuum tube; -a v4movable -tuuing elementin said resonator andhaving sliding pressure seals with the walls of said resonator; a sourceof hydraulic liquid, two iconduits, :from said fsource of liquid torespective; oppositezsidesof said ,tuning felementzin saidrresonator.,and means .to vary the liquid :pressures-differentially in said ttwoconduits to move said tuning=ele.

ment.

References Cited in fheiile of this patent UNITED STATES PATENTS

